High-purity silicon is generally produced in a multistage process starting from metallurgical silicon which can have a relatively high proportion of impurities. To purify the metallurgical silicon, this can, for example, be converted into a trihalosilane such as trichlorosilane (SiHCl3) which is subsequently thermally decomposed to give high-purity silicon. Such a procedure is known, for example, from DE 29 19 086. As an alternative thereto, high-purity silicon can also be obtained by thermal decomposition of monosilane, as described, for example, in DE 33 11 650. Monosilane can be obtained, in particular, by disproportionation of trichlorosilane. The latter can in turn be prepared, for example, by reaction of metallurgical silicon with silicon tetrachloride and hydrogen.
To accelerate the disproportionation, it is possible to use catalysts. Basic catalysts such as the amine compounds known from DE 25 07 864 and derivatives have been found to be particularly useful. These are preferably used in bound form, as described, for example, in DE 33 11 650. Catalysts bound to solid supports can be separated in a simple manner from liquid or gaseous reaction mixtures. In the case of amine compounds, introduction of contaminated amines into the silane/chlorosilane mixture can be avoided in this way. Owing to the associated advantage, virtually only amine catalysts immobilized on supports or amine catalysts incorporated into crosslinked polymers are nowadays used in the industrial disproportionation of trichlorosilane.
It is known from, inter alia, DE 198 60 146 that disproportionation of trichlorosilane can be allowed to proceed according to the principle of reactive distillation. Reactive distillation is characterized by a combination of reaction and separation by distillation in one apparatus, in particular in a column. In this apparatus, the lowest-boiling component is continually removed by distillation, with maintenance of an optimal difference between equilibrium state and actual content of low-boiling components or lowest-boiling component always being strived for in each volume element of the apparatus.
The advantages of reactive distillation can be combined with the advantages of catalyzed reaction of trichlorosilane. This can be achieved by carrying out the disproportionation, for example, of trichlorosilane into silicon tetrachloride and monosilane, in a column in which the packings (packing elements, internals, etc.) which make mass transfer possible are combined with catalytically active solids. In particular, such a column can contain a catalytically active solid as packing elements.
It is naturally necessary to take into account the thermal stability of the catalytically active packing elements used. In general, these are based on polystyrene-divinylbenzene resins which are commercially available and relatively inexpensive but become unstable at temperatures just above 100° C. In principle, disproportionation of trichlorosilane can be accelerated to a greater degree with higher reaction temperatures. In practice, however, a compromise has to be made because of the limited thermal stability of the catalyst.
It could therefore be helpful to develop and improve known processes for disproportionation of trichlorosilane, in particular in respect of the rate of the reaction of trichlorosilane.